When I first started all grain the guy at my LHBS told me that once opened, they'll keep for about 6 months in the fridge. I seem to remember someone on here saying that their strength fades over time, but to what extent I'm unsure.
I have a packet of Chinook that seems to be never ending, it's been open a few months and still has a very strong smell and seems to be still doing its job properly.